staging: rtl8723au: Add warning about this driver being deprecated

This driver is deprecated and superseded by rtl8xxxu. It will be
removed in a future kernel release.

Add a warning to Kconfig and at device init time to notify users and
allow them time to switch over.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>